1. Field of the Invention
The present invention relates to a UHV breaker used in a UHV electric plant such as the substation on a million-volt power supply system line and, more particularly, a resistor-provided UHV breaker wherein resistor contacts for allowing making and breaking to be achieved through resistors are connected parallel to main contacts for allowing making and breaking to be achieved through no resistor.
2. Description of the Related Art
The resistor-provided breaker has been well-known as being suitable for use in a substation on a 500,000 volt power supply system line. In the case of this type of breaker, the making of a resistor is made about 10 ms before the making of a main contact to suppress overvoltage. When the priority making of the resistor is made in this manner, energy consumed by the resistor is about several Mjoules. The resistor is not so large in volume, length, and weight, as compared with those of the main contact. This small-sized and light-weight resistor, therefore, can be arranged parallel to the main contact and both of its ends can be fixed to the main contact by fixing members such as bolts. When it is fixed in this manner, it can have a high mechanical strength.
Recently, voltage supplied through the power transmission system line is made higher and higher and a power supply system line through which a ultra high voltage such as 1 million volts is supplied is now being planned. The UHV breaker intended to be used on this UHV power supply system line sometimes employs a main contact unit comprising two main contacts. In order to make the breaking easy, a resistor is sometimes inserted parallel to the main contact at making and even breaking times. The rise of recovery voltage caused in the main contact after the breaking is thus reduced. Further, the resistor is sometimes inserted parallel to the main contact to suppress overvoltage caused after the breaking of earthed current, for example. This resistor needs a resistor contact because it must be shut off from the main circuit 30-40 ms after the breaking is finished. As seen in the case of the 500,000 volt breaker, the resistor contact is closed at the making time. Overvoltage is thus suppressed by the resistor before the making of the main contact.
Energy which must be absorbed by the resistor of the resistor-provided UHV breaker becomes 20-30 times larger, as compared with energy absorbed by the resistor of the 500,000 volt breaker. The resistor which must be used by the resistor-provided UHV breaker to absorb this large energy becomes so large in volume, length, and weight, as compared with those of the resistor used by the 500,000 volt breaker. Particularly to obtain a certain voltage-resistant capacity, the resistor of this type must be made by piling a plurality of disk-like resistant elements one upon another. This makes the resistor quite longer than the conventional ones. The resistor is thus made larger in volume and heavier in weight. When it becomes quite larger, longer and heavier, it cannot have a high mechanical strength even if its both ends are fixed by a fixing member as seen in the conventional cases.
It is supposed that the resistor-provided UHV breaker becomes larger in size as compared with the 500,000 volt breaker. As the size increases with the UHV breaker, it is also supposed that the dimensional displacement of the UHV breaker becomes larger as the temperature rises. When the dimension of the resistor is changed, in this case, in the axial direction of the resistor by thermal expansion caused at the time of temperature rise, large stress is sometimes added to the resistor and support insulators by which the resistor is supported. This causes the resistor to be broken and the support insulators to be damaged.